Alkaline pulping using gaseous oxygen



Patented Mar. 23, 1954 ALKALINE PULPIN G USING GASEOUS EN George C.Harris, Wilmington, DeL, assignor to Hercules Powder Comp a corporationof Delawa No Drawing. Application Serial No. 188

6 Claims. 1

This invention relates to a pulping process for cellulosic rawmaterials. More particularly, the invention is a pulping process whereinhigh yields of substantially pure holocellulose are obtained and whereinthe ligneous and resinous materials present in the cellulosic rawmaterial may be simultaneously recovered in a commercially valuableform.

Cellulose isfound in nature associated with noncellulosic carbohydratematerials, lignin, resins, fatty acids, and various other impurities.The naturally occurring mixture of cellulose and carbohydrates, orhemicellulose, is called holocellulose. Many different pulping processeshave been proposed and used in the removal of the lignin, resins, andother impurities from these naturally occurring cellulosic rawmaterials. Most of the methods known result in considerable attack onthe cellulose itself and in the extraction of the carbohydrate materialsnormally associated with the cellulose. Some of the processes arereasonably effective in removing lignin, but are comparativelyineffective in removing the associated resinous materials. Consequently,the art has continually sought a pulping process which is more specificin its removal of lignin and resins and which solubilizes less of thehemicellulose during the digestion procedure.

An object of the present invention, therefore, is a process by whichligneous and resinous materials can be substantially completely removedfrom raw cellulosic materials.

A further object of the invention is a process for pulping rawcellulosic materials which gives a high yield of substantially pureholocellulose;

An additional object of the invention is a pulping process in which itis possible to obtain simultaneously high yields of substantially pureholocellulose and recover the ligneous and resinous materials in acommercially valuable form;

Generally described, the present invention is a process forsimultaneously pulping cellulosic raw materials and removingtheimpurities therefrom which comprises digesting the cellulosic rawmaterial in an aqueous medium maintained at a pH of from 7 to 9; saiddigestion being conducted at a temperature of from l25-175 C in anatmosphere of pressurized oxygen-carrying gas, said oxygen-carrying gashaving, at reaction temperature, a partial oxygen pressure of at least800 p. s. i. If pure oxygen is employed,

the minimum pressure at reaction temperature is 800 p. s. i. If anoxygen-carrying gas such air is employed, then a pressure must be emany,Wilmington, DeL,

October 4, 1950, ,487

2 ployed which will result in sure of at least 800 According to of theinvention, the

a partial oxygen presp. s. i. at reaction temperathe preferredembodiment digestion is carried out at a pH of from 7 to 7.5, at atemperature of about (3., and in" an atmosphere of pure oxygen at apressure of 1000-1100 p. s. i. alkaline reagent is sodium bicarbonate.

No reagent penetration period is necessary to prevent degradation of thecellulose or hemicellulose in the process of the invention althoughminor improvements in yield may possibly result with a few rawmaterials. As will be seen, the unbleached pulp from the presentprocess'is com-- parable in strength to bleached sulfite pulps. The pulpobtained from the process of the invention may be bleached byconventional processes to producea pulp of superior characteristics;

As will be seen from the subsequently presented examples, it has beenfound that several ele' ments of the process in accordance with theinvention are essential to operability. In the first place,substantially no pulping is obtained if, during the reaction, the pH ofthe medium is below '7; i. e., if the reaction medium is acid, unlessvery high temperatures are employed for long periods of time However,the pulp thus obtained is too degraded to be of any value. Pulping isobtained at all pHs above 7 but the reaction becomes more drastic withhigher pl-E's until at pHs of over 9, the oxygen used in the reactionattacks the cellulose and other carbohydrate materials as well as thelignin and resins. The result is that highly degraded celluloses: areobtained in poor yields if the pH is over about 9; In fact, if the pH israis 9, it is possible to solubilize wood and other cellu-losic rawmaterials completely on: treatment with oxygen under the conditions ofpressure and temperature employed in the process of the invention.Furthermore, the pH must not only be within the range of 7 to 9 at thebeginning of the operation, but must be maintained on the alkaline sidesubstantially throughout the cook in spite of the formation of acidicmaterials by the oxidative degradation of the lignin. It will beappreciated that if the end pH of the reaction liquoris only veryslightly on the acid side; e. g., in the order of 6.8 or 6.9, thereaction medium has been alkaline for substantially the entire cook inaccordance with the invention. However, the reaction liquid must besubstantially neutral at the end of the reaction so that the cock is notconducted under acid conditions for any substantial period. A bufferingagent must,

The preferred ed substantially above therefore, be present or a similarbuffering action must be obtained by the continuous addition of analkaline agent to maintain the pH within the necessary range. Ingeneral, the salts of a strong base and a weak acid are preferred whenused so that. the reaction solution has a pH of between 7 and 9. Suchsubstances include sodium bicarbonate, sodium tetraborate, sodiumbenzoate, sodium hydrosulfide, sodium phenolate, sodium acid tellurate,and sodium monohydrogen orthophosphate. Alkali metal salts of the weakacids are preferred. Pulping can be obtained when alkaline earth metalsalts and higher valence metal salts are used, but because these metalsform insoluble salts with constituents of the solubilized lignin andother noncellulosic materials and because these salts remove from theresulting pulp, the use of such alkaline earth and other higher valencemetals is undesirable except when certain special pulps are needed.

In order to obtain pulping, it is also essential that gaseous anddissolved oxygen be present. The oxygen may be in a pure state or inphysical admixture with another gas as in the case of air. Substantiallypure oxygen is preferred. The

1 oxygen or oxygen-carrying gas should be introduced into the reactionvessel at a. pressure which will result in a partial oxygen pressure ofat least 800 p. s. i. at temperature. If the necessary pressure is notemployed, the reaction rate is lowered to a point where the timenecessary for pulping is undesirably increased. Under these conditions,the pulp is maintained at a high temperature for such an extended periodthat a degraded product is obtained. Consequently, if the pressurizedoxygen or oxygen-carrying gas is admitted to the digester prior toraising the temperature, the gas should have a partial oxygen pressureof about 500 p. s. i.

The temperature of the reaction must be maintained between 125 C. and175 C. Below 125 C., the reaction rate is too low and poor yields ofdegraded pulp are obtained. Above 170 C., a highly degraded pulp isobtained. The reaction time of the process of the invention is fromabout 2 to about 5 hours as compared with a minimum of about 8 hours forany known process resulting in acceptable yields of high-grade pulp.

The reaction rate is also affected by the degree of agitation employed.Faster reaction times are obtained with increases in agitationefficiency. Satisfactory agitation may be obtained by mechanical meansor by liquor circulation.

The amount of water employed is not critical but is governed bypractical considerations. Enough water must be employed, of course, toproduce a workable slurry. The use of an excessive.amount above thispoint necessitates the use of larger reaction vessels for a given chargeand complicates the recovery of the reaction products and reagents. Itis preferred, therefore, that water to raw cellulosic material ratios bekept between 6:1 and 20:1.

Having generally described the invention, the following examples aregiven for purposes of illustration.

EXAMPLE 1 Three hundred g. (dry weight) of solvent-extracted pine stumpshreds, 150 g. of sodium bicarbonate, and 6 liters of water were placedin an autoclave and oxygen was introduced until a pressure of 750 lbs.was reached. The contents are difficult to Hemicellulose (soda soluble)do of the autoclave were agitated and heated to a temperature of C. overa period of one hour. The pressure within the autoclave at 140 C. wasabout 1000 p. s. 1. At the end of 8 hours, during which time thecontents of the autoclave were continuously agitated and maintained at140 C., the contents of the autoclave were cooled to 70 C. The end pH ofthe reaction solution was 7.2. The resulting suspension was filtered.One hundred forty-eight g. (dry weight) of pulp was obtained for a yieldof 49.5%. The pulp exhibited the following characteristics:

A. C. S. viscosity (5%) sec 693 a-Cellulose percent 82.4 Hemicellulose(soda soluble) do 40.0 Pentosans do 3.2 Lignin do 1.3 Gums and resins(alcohol-benzene extraction) percent 0.14

EXAMPLE. 2

Fifty g. (dry weight) of solvent-extracted pine stump shreds, 12.5 g. ofsodium bicarbonate, and 1 liter of water were placed into an autoclaveand suflicient oxygen was introduced to give a pressure of 1000 p. s. i.at a temperature of 175 C. The contents of the autoclave were maintainedat 175 C. with agitation for a period of hour. The end pH of thereaction solution was 4.8. When the resulting suspension was filtered, a38% yield of very highly degraded pulp was obtained which had thefollowing characteristics:

A. C. S. viscosity (5%) sec 2 a-Cellulose "percent--. 53.5 Pentosans do6.2 Lignin do 0.5

EXAMPLE 3 Three hundred g. (dry weight) of spruce chips, g. of sodiumbicarbonate, and 6 liters of water were introduced into an autoclave.The procedure of Example 1 was repeated except that the contents of theautoclave were maintained at 140 C. for 4 hours instead of 3 hours. Theend pH of the reaction solution was 7.0. One hundred eighty-one g. (dryweight) of pulp was obtained for a yield of 60%. The pulp exhibited thefollowing characteristics:

A. C. S. viscosity (5%) oc-C611111OS8 percent sec 977 81.4 36.2 5.8 1.5

Pentosans do Lignin do Gums and resins (alcohol-benzene extraction)percent EXAMPLE 4 A. C. S. (viscosity (5%) sec iii-Cellulose percentHemicellulose (soda soluble) do Pentosans do Lignin do Gums and resins(alcohol-benzene extraction) "percent...

Similar runs were then made using lower concentrations of sodiumbicarbonate to give a pH on the acid side. In each case, unreacted woodwas recovered showing incomplete reaction. As illustrated in Example 2,higher temperature cooks under acid conditions give almost completelydegraded pulps.

EXAMPLE Three hundred g. (dry weight) of aspen chips, 150 g. of sodiumbicarbonate, and 6 liters of water were introduced into an autoclave.The procedure of Example 1 was repeated except that the reaction time at140 C. was 4 hours. The end pH of the reaction solution was 7.1. Onehundred seventy-eight g. (dry weight) of pulp was obtained for a yieldof 59%. The pulp exhibited thefollowing characteristics:

A. C. S. viscosity (5%) sec 1833 a-Cellulose -per cent 79.7Hemicellulose (soda soluble) do 30.5 Pentosans do 18.5 Lignin do 0.9Gums and resins (alcohol-benzene extraction) per cent 0.17

EXAMPLE 6 Fifty g. (dry weight) of aspen chips, 25 g. of sodiumbicarbonate, and 500 cc. of water were charged into an autoclave andcooked for 2 hours at a temperature of 150 C. and under an oxygenpressure of 800 p. s. i. The end pH of the reaction solution was 8.90. Apartially degraded but acceptable pulp was obtained in 50% yield whichhad an A. C. S. viscosity (5%) of 212 seconds.

In a similar run with pine chips in which aqueous sodium hydroxide wasemployed to give a pH of 9.3, the charge was almost completelydissolved. Only an 18% yield of very highly degraded pulp was obtained.

EXAMPLE 7 Three hundred g. (dry weight) of spruce chips, 150 g. ofsodium bicarbonate, and 3 liters of water were charged into an autoclaveand cooked with agitation for 4 hours at 120 C. and under an oxygenpressure of 1000 p. s. i. The end pH of the reaction solution was 7.4.At the end of this time, only a small amount of pulp had been produced.Longer reaction time under these conditions resulted in greater yieldsbut with undesirable degradation of the pulp. When the tem perature wasraised, increased yields of good quality pulp were obtained in normalreaction times.

EXAMPLE 8 Fifty g. (dry weight) of spruce chips, 25 g. of sodiumbicarbonate, and 500 cc. of water were charged into an autoclave andcooked for 2 hours at a temperature of 175 C. under an oxygen pressureof 1000 p. s. i. The end pH of the reaction solution was 7.7. A 37%yield of partially degraded but acceptable pulp was obtained which hadan A. C. S. viscosity (5%) of '70 seconds and an a-CBIIHIOSG content of59%. The use of a longer reaction time at 175 C. or the use of highertemperatures for the same period resulted in a highly degraded pulp.

EXAMPLE 9 Three hundred g. (dry weight) of solvent-extracted pine stumpshreds, 150 g. of sodium bicarbonate, and 3 liters of water were chargedinto an autoclave and cooked for 3 hours at a temperature of C. andunder an oxygen pressure of 800 p. s. i. A somewhat degraded butsatisfactory pulp was obtained in 55% yield which had an A. C. S.viscosity (5%) of 385 seconds).

EXAMPLE 10 The run described in Example 9 was repeated under identicalconditions except for employing an oxygen pressure at temperature ofonly 500 p. s. i. A 5% yield of partially degraded pulp was obtainedwhich had an A. C. S. viscosity (5%) of 220 seconds. A much longerreaction time under these conditions increases the yield somewhat butresults in a highly degraded pulp.

EXAMPLE 1 1 Three hundred g. (dry weight) of spruce chips, 300 g. ofsodium tetrabcrate, and 3 liters of Water were introduced into anautoclave. The procedure of Example 1 was repeated with A. C. S.viscosity (5%) sec 461 a-CEHUIOSG per cent 72 Hernicellulose (sodasoluble) do 38 Pentosans do 5.9 Lignin d0 2.0

Gums and resins (alcohol-benzene extraction) per cent-.. 0.14

EXAMPLE 12 Gums and resins (alcohol-benzene extraction) per cent 0.08

EXAMPLE l3 Handsheets were prepared from unbleached pulps produced inaccordance with the invention and compared with sheets prepared fromBurgess bleached sulfite .pulps produced from similar raw materials. Thesheets compared were made from pulps beaten to a comparable freeness.The sheets made from the pulp produced according to the invention werefound to possess superior dry burst and tensile strengths. Their drytear strength was somewhat lower than the dry tear strength of thesheets prepared from the Burgess bleached sulfite pulp but wasrelatively high for a paper having such extremely high bursting andtensile strengths. set forth in Table I. Basis weights were 40 1b.:1.0.All tests were conducted in accordance with TAPPI specifications.

The results of these tests are 'at pH 2.0 and a consistency of 3.0%

citation Purity of 3.3%

Aspen pulp prepared by the process of this invention was bleached in athree-stage process consisting of the following steps: (a) chlorinationwith 2.3% chlorine based on pulp weight for 0.5 hour at 24 C., (b)extraction with 0.25% sodium hydroxide at a consistency of for one hourat 96 C., (c) alkaline bleach with 1.0% chlorine (hypochlorite at pI-I9.8) in 10% consistency for one hour at 55 C. The bleached pulp was abright white and was characterized by an Exand a dominant wave length of576.5 millimicrons with respect to Illuminant C.

The bleached pulp was refined as follows: It was treated with 11.1%sodium hydroxide at a consistency of 10% for 2 hours at 0 C. The producthad an a-cellulose content of 92.8%. The yield of this product is 29% onthe aspen chips, 52% on the unbleached pulp, and 54% on the bleachedpulp.

EXAMPLE Samples were taken from the reaction liquors obtained in thepulping of softwocds and hardwoods by the alkaline oxidation process ofthe invention. The individual samples were partially evaporated and thenacidified to precipitate the ligneous and resinous compounds solubilizedin the particular process. These materials were analyzed with thefollowing results:

Table II Table I Air Re- Bur Dry Freeness Dry Tear sistance Designation(lb./ Tensile cc. S.R. (g./sheet) (sec/ sq. in. (lb./1n.) cc'lscb m)Burgess bleached su1fite. 840 14. 9 78. 6 13. 8 4 Alkaline oxidationpulp 840 28. l 51. 4 21. 8 4 Burgess bleached sulfite 400 23.0 40. 1 26.8 700 Alkaline oxidation pulp. 350 37. 5 41. 5 32. 7 930 Burgessbleached sulfite 300 22. 0 26. 0 1, 492 Do 27. 4 25. 5 Alkalineoxidation pulp 160 31.8 23. 9 30. 1 16 hrs.+

1 Contained 3% K gum size (sodium salt of K gum rosin). EXAMPLE 14 15 tothe carboxylic acid groups produced on oxidative degradation of thelignin molecule. The acidity of the ligneous substances recovered fromsulfite liquors is due to some phenolic acidity and the sulfo groupsintroduced into the large, undegraded lignin molecule. The acidity ofthe lig'neous material in the sulfate liquor is entirely due to phenolicacidity. In all cases, the alkaline oxidation process of the inventionproduces more highly acidic andmore highly hydroxylic solubilizednoncellulosic material of greatly decreased molecular weight. Asillustrated, this decreased molecular weight results in solubiltycharacteristics which give the materials commercial value.

The uses to which the solubilized materials recovered from the alkalineoxidation liquor can be put are mainly due to the greatly enhancedsolubility, high acidity, and high alcoholic hydroxyl contentI Completeesterification with low molecular weight acids and alcohols produces lowmelting substances usable as plasticizers. Intermolecular condensationwith polyhydroxy and polycarboxylic materials results in high meltingresins usable in protective coating finishes of all sorts where color isnot a factor. Y J The solubilized materials in the alkaline oxidationreaction liquor may be recovered in a number of ways. The resins andlignin are present in the liquor as soluble salts and are most easilyrecovered by partial evaporation and acidification as illustrated inExample 15. It is also possible Alkaline Oxidation Hardwood SoftwoodAcid No Methoxyl, percent Hydroxyl, percent Hydroxyl corrected for acidnumber Molecular wt Sulfate, Softwood Solubilities in- Acetone.. SolubleSoluble Insoluble. Methanol Partially soluble Partially s0luble Do.Chlorinated hydrocarbons (0112012), Soluble Soluble DO.

(Cl-I013), 014). Dilute alkali do do.- Soluble. Hydrocarbons (aliphaticand aromatic) Insoluble Insolub1c Insoluble.

1 Although it is accepted that the insolubility of the ligneouscompounds which are by-products of the various pulping processes is dueto their high molecular weights, there are the molecular Weight ofthebasic lignin is widely believed, however,

no definite molecular materials include softwoods, semihardwoods,hardwoods, bamboo, straw, hemp, jute, and fiax seed.

tion be limited only by claims.

What I claim and desire to protect by Letters Patent is:

1. In the process for the production of substantially lignin-freeholocellulose from cellulosic raw material by pulping the material,under pressaid pulping, whereby a high yield of the substantiallylignin-free holocellulose is obtained and the lignin and resinsrecovered molecular weight and good solubility characteristics.

2. The process in accordance with claim 1 in which the oxidizingmaterial essentially consists of substantially pure oxygen.

3. The process in accordance with claim 1 in which the oxidizingmaterial essentially consists of air.

4. The process in accordance with claim 1 in l which the alkalinesolution essentially consists of sodium bicarbonate.

5. The process in accordance with claim 1 in which the alkaline solutionessentially consists of sodium tetraborate.

6. The process in accordance with claim 1 in which the alkaline solutionessentially consists of sodium monohydrogen orthophosphate.

GEORGE C. HARRIS.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 21,077 Lyman Aug. 3, 1858 1,831,032 Richter Nov. 10, 19311,902,916 Strecker Mar. 28, 1933 1,996,797 Dreyfus Apr. 9, 19352,022,654 Dreyfus Dec. 3, 1935 2,234,188 Morgan Mar. 11, 1941 2,243,050Plant May 20, 1941 2,516,827 Marshall at al July 25, 1950 2,538,742Willey Jan. 16, 1951 FOREIGN PATENTS Number Country Date 312,618 GermanyMay 31, 1919 185,421 Great Britain 1924 238,305 Great Britain Aug. 17,1925 284,846 Great Britain Feb. 9, 1928 271,524 Great Britain Mar. 29,1928 559,405 Germany Sept. 20, 1932 815,651 France July 20, 1937 OTHERREFERENCES Ser. No. 318,386, F'reudenberg et al. (A. P. 0.), publishedApril 20. 1943.

1. IN THE PROCESS FOR THE PRODUCTION OF SUBSTANTIALLY LIGNIN-FREEHOLOCELLULOSE FROM CELLULOSIC RAW MATERIAL BY PULPING THE MATERIAL,UNDER PRESSURE, WITH ALKALINE SOLUTION AND IN THE PRESENCE OF ANOXIDIZING MATERIAL AT ELEVATED TEMPERATURE, THE IMPROVEMENT COMPRISINGPULPING THE SAID MATERIAL WITH AN ALKALINE SOLUTION OF AT LEAST ONEMATERIAL OF THE GROUP CONSISTING OF SODIUM BICARBONATE, SODIUMTETRABORATE, SODIUM BENZOATE SODIUM HYDROSULFIDE, SODIUM PHENOLATE,SODIUM ACID TELLURATE, AND SODIUM MONOHYDROGEN ORTHOPHOSPHATE AT ATEMPERATURE BETWEEN 125* AND 175* C. FOR A PERIOD FROM ABOUT 2 TO ABOUT5 HOURS IN THE PRESENCE OF AT LEAST ONE OXIDIZING MATERIAL OF THE GROUPCONSISTING OF SUBSTANTIALLY PURE OXYGEN AND AIR HAVING A PARTIAL OXYGENPRESSURE OF AT LEAST 800 POUNDS PER SQUARE INCH WHILE MAINTAINING THEALKALINE SOLUTION AT A PH OF BETWEEN 7 AND 9 THROUGHOUT SAID PULPING,WHEREBY A HIGH YIELD OF THE SUBSTANTIALLY LIGNIN-FREE HOLOCELLULOSE ISOBTAINED AND THE LIGNIN AND RESINS RECOVERED FROM SAID SOLUTION HAVE LOWMOLECULAR WEIGHT AND GOOD SOLUBILITY CHARACTERISTICS.